If your External HDD is a WD Passport, it has extra hardware that does the encryption part. The data transfer is slow probably due to improper handling like a fall or pulling off the Ext HDD while iti s still running and hasn't spinned down. (multiple times)

If the problem has arisen due to improper handling, the only probable solution is to send it for data recovery but the cost involved can be extremely high. You have to come to a conclusion regarding the importance of the data to you if you finally plan to send the External Hard Disk for Data Recovery.Otherwise...you will have to wait for a lifetime to get your data which is actually impractical.

Resources

Repairability

Stay in the loop

It's time to speak out for your right to repair

We have a chance to guarantee our right to repair electronic
equipment—like smartphones, computers, and even farm equipment. This is a once-in-a-generation
chance to protect local repair jobs—the corner mom-and-pop repair shops that
keep getting squeezed out by manufacturers.

Join the cause and tell your state representative to support Right to Repair. Tell them you believe repair should be fair, affordable, and accessible. Stand up for your right to repair!